Credit: CourtesyCAN’T MISS KID: Jayson Tatum knocks down a 3-pointer during the Celtics’ 108-83 rout of the Cleveland Cavaliers in Game 1 of the Eastern Conference finals Sunday at the Garden.

Semi Ojeleye wasn’t so much told, as he was asked, to cover LeBron James on Sunday.

In this case maybe less familiarity with the monstrosity of the task is better, and best filled by a younger player, or in the case of Ojeleye, a rookie.

That, in a nutshell, is what characterizes these younger Celtics, on a roster with an average age of 26, not including two-way contract players Jabari Bird and Kadeem Allen.

“It’s been spectacular playing with this group, and you can actually feel the energy, the youth, the excitement,” 22-year-old Jaylen Brown said after yesterday’s practice. “But we’ve been playing a lot older than our age, I think — Terry Rozier’s (low) turnovers, turnover ratio, JT’s tough shot-making ability. Lot of the rooks stepping up. Semi Ojeleye asked to guard LeBron. Gotta grow up quick in this league.”

JT is Jayson Tatum, the 20-year-old rookie who always seems better in the second half than the first, just as he’s playing better now than at midseason.

“Really don’t know any better in my first season,” said Tatum. “Obviously understand what’s at stake, and keep going. You just have to keep going, keep playing.”

Ah, to be young and naïve.

Coach Brad Stevens raised that point about his wise mix of veterans and youngsters this season. For all of the experience passed down from Al Horford, Kyrie Irving and Aron Baynes, there’s the other end of the scale with Tatum, Brown, Rozier and, at an especially seasoned 24, Marcus Smart.

Stevens’ point, that sometimes it’s beneficial to go into a series against a veteran group like the Cavaliers without the emotional luggage and memory of an older team, gains clarity with each game.

The phrase often mentioned is “house money.” Thus far, now against a Cavaliers team with an average age of 30, there’s a lot of that currency to go around.

“You think about it, everybody has counted us out from the beginning of the season. We’ve been playing with house money ever since,” said Smart. “We have nothing to lose. Every time we go up against somebody we’re the underdogs.”

That’s why Stevens’ theory of naivete resonates.

“You hear it, you feel it, you keep that inside and let it drive you,” said Smart. “Every team we’ve played in the playoffs has kind of counted us out, so when you’re on a team with nothing to lose that can play carefree, that’s a dangerous team. I can see that teams overlook us because of our age, and that works to our advantage when they take us lightly. Then it comes as a surprise to ‘em. We’re just ready.”

At 29, Marcus Morris loves the approach younger teammates have taken to attacking the void left by season-ending injuries to Irving and Gordon Hayward.

“Everybody is looking at it as an opportunity, with nothing to lose,” said Morris. “At the end of the day we’re playing with house money, from what I can see. Nobody expected us to be here. With nothing to lose, every day we’re going out at 110 percent. It’s freedom. It depends on who you are. If you approach the game worrying about who is on the other side, you’ve lost that battle already. Just go out there, play and compete.”

Game 2 is tonight, presumably with a more determined Cavs team on the floor. And once again Celtics Youth will have a short memory on its side.

“I believe that,” said Brown. “Maybe we’re just young enough to not care about what society says the norm should be. End of the day you just have to come out and play. You could say we’re not affected by it, and then come out and get beat by 20 points tomorrow. People can say whatever they want, but you have to make it true.”